This invention relates generally to a laminated article of manufacture and a method of making wherein at least one layer of the laminate comprises at least one rare earth element. More particularly, this invention relates to electrospark depositing a rare earth coating on a metal substrate wherein the coating may be subsequently bonded to another metal substrate. Still more particularly, this invention relates to electrospark depositing the rare earth element erbium on a zirconium alloy substrate that can be subsequently formed into a fuel assembly component for neutronic control in a light water reactor. xe2x80x9cLaminatedxe2x80x9d herein is defined as composed of layers of metallurgically-bonded material with at least one substrate layer and at least one coating layer. xe2x80x9cRare earthxe2x80x9d element is defined in the conventional manner, that is, an element of the lanthanide series.
The operation of a nuclear power plant requires that the reactor core maintain criticality throughout the duration of its operating cycle. In order to operate for an extended period of time, the reactor core must initially have excess reactivity (i.e., an excess amount of fissile material). This excess reactivity changes over time such that by the end of its operating cycle, the excess reactivity approaches zero, and the reactor core can no longer remain critical. At this point, the reactor is shut down and the core is refueled.
The amount of excess reactivity in a reactor core is limited to maintain a safe, controlled nuclear chain reaction. The primary method of reactivity control is to fuel the reactor core with a number of fuel xe2x80x9cbatches,xe2x80x9d each batch generally having been operated a cycle more than the succeeding batch. Ideally, the reactivity of each batch would be designed such that the average of the whole core allows the core to be just critical. When a particular fuel batch does not have sufficient reactivity to meaningfully contribute to the excess reactivity of another fuel cycle, the batch is discharged to the spent fuel pool and a fresh fuel batch takes its place.
Because fresh fuel must typically last for xcx9c1200-2000 effective full power days in the reactor (depending on the particular cycle design), fresh fuel must be loaded with far more reactivity than would be required if the fresh fuel only needed to last for 1 cycle. These high levels of excess reactivity require design measures to maintain the reactor core within acceptable safety margins. One of these design measures is the incorporation of a burnable neutron absorption material (sometimes called xe2x80x9cburnable poison,xe2x80x9d referred to hereinafter as simply xe2x80x9cabsorberxe2x80x9d) within the fuel assemblies that provide xe2x80x9cnegativexe2x80x9d reactivity to the batch in an amount that is able to help control the excess reactivity as the reactor cycle proceeds (U.S. Pat. No. 5,241,571, No. 5,267,290, and No. 5,872,826, referred to hereinafter as patents ""571, ""290, and ""826, respectively).
Absorbers typically comprise one or more of the following high neutron absorption cross-section elements: boron, cadmium, silver, indium, hafnium, and the rare earth elements of gadolinium, erbium, and samarium. Some of these absorbers have been incorporated in xe2x80x9cdiscretexe2x80x9d absorber pins that occupy fuel pin lattice positions in a fuel assembly, as a coating on fuel pellets, as a constituent of the fuel, and as an alloying element of a component of the fuel assembly (e.g., fuel cladding or structural members). All of these methods, however, have shortcomings. For example, discrete absorber pins and absorber-containing fuel displace power-producing fuel, operate at lower linear heat generation rates than standard fuel pins, and require more stringent controls in material handling and fabrication during fuel assembly manufacture. Furthermore, the alloying approach restricts the range of options available to the designer for choosing the optimum amount and spatial distribution of the absorber within the fuel assembly component to meet reactivity needs.
A more attractive and versatile approach is provided by patent ""826 which discloses a fuel assembly design comprising absorbers as sheets that are embedded in the structural channel box of a fuel assembly using a variety of encapsulation, rolling, and pressing techniques. Such an approach provides flexibility in the amount and location of the absorber within the fuel assembly and keeps the absorber from directly contacting the reactor coolant. In addition, this approach offers a method to increase fuel burnup. By replacing the absorber-containing structural member (e.g., guide tube, channel, duct) of a fuel assembly during a reactor shutdown with another member containing a lower amount of absorber (without replacing the fuel pins) and reinserting the assembly in the reactor, fuel lifetime can be increased. The absorber sheets disclosed in patent ""826 were made of cadmium, samarium, boron, gadolinium, silver, indium and hafnium.
An optimum reactivity profile for each fuel assembly would be one that has a flat reactivity curve throughout its life and then drops off to zero just prior to the assembly being discharged. Practically, this would require that the negative reactivity of the absorber in the assembly burn out at exactly the same rate as the fissile fuel, and that all of the absorber is depleted at the end of the cycle. Any absorber that remains at the end of the life of the fuel assembly contributes to a residual negative reactivity that can shorten assembly (and therefore core) life. In practice, it is very difficult to achieve a flat reactivity curve with no absorber left at the end of assembly life. Each absorber has its own nuclear characteristics, and every reload batch is a compromise between competing alternatives.
In this regard, the designer has two considerations to achieve the compromise in designing a core load. First, any residual negative reactivity from absorber that remains at the end of assembly life results in a loss of economic value of the assembly. There is no way to mitigate the presence of residual negative reactivity except to add more fissile material to the initial fuel load. Clearly, the best designs would minimize residual negative reactivity. Second, the amount of fuel assembly excess reactivity controlled by the absorber during the life of the assembly may be highly variable. This is because there are a variety of methods that can be used to control overall core reactivity, including control rods, water flow, etc. In addition, there is generally sufficient thermal margin in fuel designs to allow reasonably wide assembly power/reactivity swings (xcx9c25%) through the life of the assembly. However, there are limits to what can be accommodated from a safety standpoint in a core design. It is clear, however, that it would be more economically desirable to design an assembly that has larger swings in reactivity than a large amount of residual negative reactivity.
FIG. 1 provides a graphical comparison of some reactivity calculations for several sample fuel assembly designs containing a variety of absorbers. The xe2x80x9cGadolinium Pinsxe2x80x9d curve is the baseline curve representing a fuel assembly designed with gadolinium mixed with fuel in several fuel pins in the assembly. This design represents the current state of the art of absorber application in boiling water reactors (BWRs). Fuel pins with gadolinium have been used for a number of years, and has provided a reasonable balance of reactivity and residual negative reactivity. The other curves shown in FIG. 1 use an absorber incorporated in the structural member of the fuel assembly. As discussed previously, incorporation of the absorber in a structural member (as an non-alloying element) has advantages over other approaches.
The common basis for each curve in FIG. 1 is that each absorber analyzed is placed in an assembly with the same initial amount of U-235. The amount of absorber is adjusted to try to obtain a reactivity curve that is as constant as possible with a peak reactivity of  less than 1.2 and a minimum reactivity  greater than 0.9. Therefore, the reactivity potential of each of the sample assemblies calculated is exactly the same. The figure clearly shows the differences that can be achieved with the different absorbers.
The use of samarium, hafnium, indium or silver, as suggested by patent ""826, would not meet the economic requirements of a fuel assembly with minimal residual reactivity. As can be seen in FIG. 1, samarium and silver clearly result in a significant residual reactivity penalty as compared to the baseline. Samarium also has the additional disadvantage of having a very large increase in reactivity at the beginning of operation, which could potentially result in operational difficulty in maintaining safety margins. Hafnium and indium are much better for this application, however, there is still a penalty relative to the baseline. The penalty would result in tens of days less operation, which would be associated with a significant cost penalty. Given the penalty of using hafnium or indium in comparison with the state of the art, there would be little economic advantage to incorporating hafnium or indium in the channel.
The incorporation of boron into the structural member, also suggested by patent ""826, would be an adequate absorber from the standpoint of residual reactivity worth. The residual reactivity at 2000 days is quite similar to the baseline. However, the reactivity fluctuations are relatively large in the first 800 days. The large fluctuation may be difficult to accommodate safely during operations. In comparison to the baseline, the use of boron would provide little operational advantage, and may make operations more difficult. Because of this, there would be no clear reason that the use of boron in the structural member would be better than the state of the art. Furthermore, boron produces helium as a result of neutron capture and may result in degradation of the structural integrity of the assembly.
As shown in FIG. 1, the use of erbium (Er) results in a relatively smooth reactivity curve. The initial and peak reactivities are similar to the baseline, however the curve is much flatter than the baseline through 800 days. This would likely result in fuel assembly designs that can maintain the safety and thermal margins that are in the current gadolinium assemblies. The real reactivity benefit, however, can be seen after 800 days when it becomes clear that the residual negative reactivity component is much less than the baseline. The cycle benefit may be on the order of 150-200 days, which could be directly transferred into fuel cycle savings either through a reduced initial enrichment requirement or through longer operating cycles. Depending on the specific neutronic requirements, further benefits may be obtained by combining the Er with other absorbers.
Support for the use of Er as the absorber is further provided by patents ""571 and ""290. Patent ""571 discloses Er as an alloying element for the zirconium-based fuel cladding or structural member of the fuel assembly. Patent ""290 discloses a coextruded fuel pin cladding having a layer of zirconium absorber alloy containing Er. Despite these positive developments in the use of Er, the approaches disclosed in patents ""571 and ""290 still have some of the shortcomings discussed earlier. It is of interest that the more attractive and versatile approach of incorporating the absorber as an embedded sheet in a structural member (patent ""826) does not teach or even suggest the use of Er.
Accordingly, there is a continuing need to incorporate an effective absorber, such as the rare earth element Er, in a structural member of the fuel assembly that overcomes the shortcomings of present methods and that improves the performance of fuel assemblies.
The present invention is a laminated article of manufacture and a method of making. The article is a structure having two or more layers, wherein at least one layer is a metal substrate and at least one other layer is a coating comprising at least one rare earth element. For structures having more than two layers, the coating and metal substrate layers alternate. In the simplest embodiment of the invention, the structure is a two-layer laminate consisting of a metal substrate (as the first layer) and a coating (as the second layer) formed on the metal substrate. The coating is formed by electrospark depositing a material on the metal substrate from an electrode comprising at least one rare earth element. In a slightly more complex embodiment of the invention, the structure is a three-layer laminate made from a two-layer laminate by the additional step of bonding a second metal substrate to the coating layer. The substrates may be any metal though reactor-grade zirconium-based alloys, iron-based alloys, and nickel-based alloys are preferred for in-reactor nuclear applications such as fuel assembly or fuel storage components. The bonding of the coating to the second metal substrate may be accomplished by hot pressing, hot rolling, high deformation rate processing (e.g., explosive bonding), or combinations thereof.
In view of the foregoing, it is an object of the present invention to bond at least one rare earth to a substrate that can be subsequently mechanically formed into a structure without flaws such as pores, cracks, or delaminations.
It is a further object of the present invention to form a laminate having a sandwiched rare earth layer that can subsequently be cold worked or hot worked while maintaining structural integrity.
It is a further object of the present invention to form a laminate having a sandwiched rare earth layer wherein the rare earth is immobile.
It is a further object of the present invention to form a low-cost, robust, and damage resistant laminate that is useful in applications requiring rare earth films.
It is a further object of the present invention to electrospark deposit material comprising Er on a zirconium alloy substrate that can be subsequently formed into a neutronic control structure for a light water reactor.
It is a further object of the present invention to increase the performance of fuel pins while maintaining or increasing the margin of safety associated with corrosion of a fuel assembly comprising the structure.
It is a further object of the present invention to increase the performance of fuel pins while maintaining or increasing the margin of safety associated with the structural integrity of a fuel assembly.
It is a further object of the present invention to decrease the handling and material control requirements during fuel assembly fabrication.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.